This invention relates to an apparatus for sealing centrifuge tubes and, more particularly, to an apparatus for sealing centrifuge tubes mounted in rotor cavities.
There is described in U.S. Pat. No. 3,998,542, issued Dec. 19, 1977 to Romanauskas et al. a cap seal for a vertically oriented centrifuge tube. Vertically oriented tubes are particularly useful in density gradient separations in which the density gradient is reoriented from vertical to horizontal and back to vertical for fractionation.
Centrifuge tube caps prior to those devised by Romanauskas et al. typically were comprised of three parts. Although quite satisfactory for most centrifuge applications, these prior art caps could not be used with vertically oriented tubes because the caps tended to be forced off by the hydrostatic pressure developed in the tubes during centrifugation.
The cap seal described by Romanauskas et al. is one in which a tapered plug is introduced into a centrifuge tube disposed in a rotor cavity having a flared mouth. A threaded disc is screwed into the open end of the tube cavity and bears axially against the plug so as to wedge the wall of the tube between the plug and the flared mouth of the rotor cavity. A somewhat similar cap seal construction, although apparently not for density gradient separations, is described in an article entitled "A Quantity Type Rotor for the Ultracentrifuge", by A. Victor Masket, Review of Scientific Instruments, May 1941, Vol. 12, Pg. 277-279.
Because of the resiliency of the tube, Romanauskas et al. provide a relatively strong, leak proof seal that is effective even under the large pressure forces which occur during the centrifugation of vertically oriented tubes. While this seal performs quite satisfactorily, as the diameter of the tubes increase, leakage can sometimes occur, particularly with larger diameter tubes, i.e., those exceeding two to three centimeters and more. The leakage problem increases as a function of the diameter of the tube, rotor speed and attitude of the tube, i.e., as the tube approaches the vertical, or is parallel to the rotational axis, the pressures exerted on the tube cap increase appreciably.
Unfortunately, as the centrifuge rotor is operated at higher speeds (typically above 50,000 rpm), the plug, which typical is formed of one of the rigid plastics, tends to be deformed by centrifugal force, that is, it is no longer round. In addition, cold flow of the centrifuge tube itself tends to cause the tube's thickness to vary. These two factors reduce the ability of the tapered plug to maintain a seal. As a result, the contents of the tube often leak causing a vacuum loss. This leakage can cause automatic shutdown of a centrifuge run, rotor imbalance and even an explosion in extreme cases. Even if the original seal is maintained at high speeds, the plug, because of its deformed shape, often permits leakage as the rotor is slowed in speed.
Accordingly, the need exists for a reliable seal for a centrifuge tube that reduces the chance of leakage, even with relatively large diameter tubes.
It is therefore an object of this invention to provide an improved seal for a centrifuge tube.
A further object of this invention is to provide an improved seal for vertically oriented centrifuge tubes.